The HIV pandemic and increases in immunocompromised patients due to cancer or organ transplants have led to a sharp rise in the number of infections by the encapsulated pathogenic fungus, Cryptococcus neoformans. This ubiquitous basidiomycete fungus enters immunocompromised individuals via the lungs and commonly infects the brain, causing meningoencephalitis and death if untreated. The prevalence of AIDS in the United States and developing countries makes a clear understanding of cryptococcal pathogenesis and improved therapeutics a public health imperative. Infection of a mammalian host is not an obligate step in the lifecycle of C. neoformans, yet this fungus has evolved mechanisms to survive in the host environment. Host tissue oxygen averages 3%, far below atmospheric levels of 21%, and levels in the brain can be as low as 1%. Thus, growth of C. neoformans in the host necessitates adaptation to low oxygen. While the requirement for virulence of environmental signal transduction pathways such as thermotolerance and nutrient starvation has been examined in C. neoformans, nothing is known about how this obligate aerobe responds to changes in environmental oxygen . Here, we will identify mechanisms by which C. neoformans senses oxygen and the adaptive gene expression programs that allow infection to progress. Preliminary experiments indicate that the membrane-bound SREBP transcription factor Sre1, which controls cholesterol homeostasis in mammals, functions in an oxygen sensing pathway and is required for low oxygen growth in C. neoformans. Given that infected host tissues are hypoxic, we hypothesize that SRE1 is required for virulence of C. neoformans. In support of this hypothesis, preliminary studies indicate that Sre1 is activated in response to low oxygen (1% - 3%) and that a C. neoformans strain lacking SRE1 shows decreased virulence in a mouse model of infection. Thus, the SREBP pathway in C. neoformans represents a potential therapeutic target for the treatment of this lethal opportunistic infection. Our specific aims are: AIM 1. To determine the requirement of C. neoformans SRE1 for virulence. AIM 2. To test the requirement of the Sre1 activating protease, Zmp1, for virulence. AIM 3. To isolate additional genes required for low oxygen growth in C. neoformans. Using a combination of gene expression profiling, genetic screening, and in vivo virulence studies in mice, we will identify novel virulence factors in C. neoformans. Our studies will provide the first information about how C. neoformans adapts to low oxygen. The HIV pandemic and increases in the number of immunocompromised patients due to cancer or organ transplants have led to a sharp rise in the number of lethal infections due to the pathogenic fungus, Cryptococcus neoformans. In this project, we will identify genes required for adaptation of this pathogen to the low oxygen environment of the host and for progression of fungal infection. These genes represent candidate targets for future anti-fungal therapy. [unreadable] [unreadable] [unreadable]